This invention is in the field of digital communications, and is more specifically directed to decoding digital signals received from wireless communications.
High-speed data communication services, for example in providing high-speed Internet access, have become a widespread utility for many businesses, schools, and homes. In its current stage of development, access to these services is available through an array of technologies. Recent advances in wireless communications technology have enabled localized wireless network connectivity according to the IEEE 802.11 standard to become popular for connecting computer workstations and portable computers to a local area network (LAN), and typically through the LAN to the Internet. Broadband wireless data communication technologies, for example those technologies referred to as “WiMAX” and “WiBro”, and those technologies according to the IEEE 802.16d/e standards, have also been developed to provide wireless DSL-like connectivity in the Metro Area Network (MAN) and Wide Area Network (WAN) context. In addition, the communications specifications resulting from the “LTE” (Long Term Evolution) project of the Third Generation Partnership Project (3GPP), which set out to improve the UMTS mobile phone standard, also promise to provide broadband data communication on a wide-area basis.
Multiple-input-multiple-output (MIMO) communication techniques have recently attracted attention, especially in the wireless network context. In a general sense, MIMO communications are realized by providing multiple signal paths between a transmitter and a receiver. In this manner, the transmitted information is either redundantly or independently transmitted as multiple spatial streams between a transmitter antennae and receiver antennae, with the multiple spatial streams combined at the receiver. The spatial diversity provided by the MIMO approach provides improved data rates for a given bit error rate in the signal. These improvements are attractive in any wireless communications context, including wireless LAN/MAN/WAN communications, and wireless telephony. An overview of MIMO technology is provided in Gesbert et al., “From Theory to Practice: An Overview of MIMO Space-Time Coded Wireless Systems”, Journal on Selected Areas in Communications, Vol. 21, No. 3 (IEEE, April 2003), pp. 281-302.
In the wireless network context, particularly in connection with the WiMAX and LTE standards, MIMO communication technology is being investigated for use in conjunction with orthogonal frequency-division multiplexing (OFDM) techniques. As known in the art, OFDM refers to a broadband communications approach in which modulated signals are transmitted over multiple narrow-bandwidth (e.g., 20 kHz) channels. The combination of MIMO and OFDM technologies is contemplated to provide excellent performance in the stringent environment of high data-rate wireless communications.
However, this combination complicates the decoding involved in order to correctly receive digital data over MIMO OFDM communications. In order to secure the MIMO wireless communications link, a MIMO decoder is necessary to resolve the incoming signal received at multiple antennae, at varying delay time. This MIMO decoder is typically realized as a maximum-likelihood (“ML”) decoder, operating at high speed and high throughput in order to provide the desired high data rate performance. Demodulation of the OFDM datastream from its multiple sub-channels requires a Fast Fourier Transform function. And because the communicated data is encoded for purposes of forward error correction, yet another decoder function is necessary at the output of the MIMO OFDM receiver. Indeed, multiple error correction decoders must be made available for the various components of the communicated datastream.
FIG. 1 illustrates the overall functional architecture of a conventional MIMO OFDM receiver system, in the example of a two-antenna system (two transmitting antennae ATX1, ATX2, and two receiving antennae ARX1, ARX2). As known in the art, conventional MIMO OFDM network devices communicate bidirectionally. For clarity of this description, only the receiver side of such a device is shown in FIG. 1; those skilled in the art will readily comprehend the manner in which duplexed transmission is realized in modern wireless communications devices.
In the example of FIG. 1, receiver system 2 includes separate “front end” circuit functions 41, 42, each including an analog front end (AFE) and a digital front end (DFE), coupled to respective receive antennae ARX1, ARX2. As well-known in the art, front ends 4 include the desired and appropriate analog and digital filtering, analog-to-digital conversion, etc. typical for OFDM receivers. Receiver system 2 also includes Fast Fourier Transform (FFT) functions 61, 62, each connected to a corresponding front end 41, 42, respectively. FFT functions 6 each demodulate the received OFDM signal. As fundamental in the art, OFDM signals are modulated over multiple subcarrier frequencies, by way of the inverse Fourier transform of a block of a datastream. As such, FFT functions 61, 62 reverse the modulation for the signals received at their respective antenna ARX1, ARX2, transforming data from the time domain (as a datastream) into the frequency domain (amplitude and phase at each subcarrier frequency).
In conventional MIMO OFDM receiver system 2, the frequency-domain demodulated signals from FFT functions 61, 62 are applied to MIMO decoder 7, which analyzes the multiple received datastreams y and recovers the transmitted signal x therefrom. As fundamental in the art, recovery of signals x1, x2 that are transmitted over two respective antennae, from signals y1, y2 received over two antennae involves the solution of a system such as:
      [                                        y            1                                                            y            2                                ]    =                    [                                                            h                11                                                                    h                12                                                                                        h                21                                                                    h                22                                                    ]            ⁡              [                                                            x                1                                                                                        x                2                                                    ]              +          [                                                  n              1                                                                          n              2                                          ]      where the hij matrix members refer to the transfer function from the jth transmit antenna to the ith receive antenna, as shown in FIG. 1 (e.g., transfer function h12 is the transfer function between transmit antenna ATX2 and receive antenna ARX1). Noise received at the ith receive antenna is represented by ni. According to this conventional example, channel estimation function 7 estimates the various transfer functions and received noise, typically from known training signals demodulated by FFT functions 6 during initialization of the link, and communicates transfer matrix estimate ĥ and channel noise estimates {circumflex over (n)}1, {circumflex over (n)}2 to MIMO decoder 8. MIMO decoder 8 in return produces datastream estimates {circumflex over (x)}1, {circumflex over (x)}2, which are estimates of the signals transmitted at antennae ATX1, ATX2, respectively, accounting for channel distortion and noise. Typically, MIMO decoder 8 is realized by way of a maximum-likelihood (ML) decoder, applied to both datastreams.
Following MIMO decoder 8, the datastream estimates {circumflex over (x)}1, {circumflex over (x)}2 are communicated to forward error correction function 10, which applies the appropriate error correction decoding to each datastream to recover the digital values of the information that was originally transmitted. As known in the art for WiMAX and LTE communications and as shown in FIG. 2, a communications link typically communicates data in frames, each frame containing a control channel 15 and a data channel 17. The contents of the control channel (15) within a frame indicate whether live data is present in the corresponding data channel (17) of that frame; as such, the validity of the signals in the data channel is dependent on the content of the control channel for that frame. According to conventional MIMO OFDM communications under the WiMAX and LTE standards, as well as others, data channel 17 and control channel 15 of each received frame are encoded differently, and are thus decoded separately. For example, a Viterbi decoder is required to decode the control information communicated in control channel 15 of the link. One or more additional decoders (e.g., “turbo” decoders, low density parity check (LDPC) decoders) are also typically required in order to decode data channel 17 in each MIMO OFDM frame. As such, in the Example of FIG. 1, forward error correction function 10 includes both turbo decoder logic circuitry 12, and also Viterbi decoder logic circuitry 14. The output from forward error correction function 10 is then applied to media access control (MAC) circuitry and functionality, as well known in the art.
As is well known in the art, each of these various multiple decoding functions involves complex logic functions and circuitry. A substantial number of logic gates and functions, and thus substantial integrated circuit “chip area”, are necessary to realize these functions, especially as data rates increase. However, the power and chip area requirements of the end systems (e.g., mobile phone handsets, laptop computer wireless access cards) into which these functions are to be implemented are also becoming more stringent. In short, the operational requirements for modern high data rate communications are becoming more complicated, while the chip area and power consumption constraints of the circuitry carrying out those functions are becoming more stringent.